i) Field of the Invention
The present invention relates to a frequency control circuit of an FM modulator, and more particularly to a frequency control circuit of an FM modulator with a small number of external parts and low number of IC pins.
ii) Description of the Related Arts
Conventionally, a multivibrator type of oscillation circuit is known. When this kind of oscillation circuit is used for an FM (frequency modulator) modulator, there must be sufficient linearity between an input voltage and an oscillating frequency. For example, in a VTR (video tape recorder), if this linearity is bad, distortion occurs in reproduced video signals which is likely to cause degradation of reproduced image quality.
In FIG. 1, there is shown a conventional, frequency control circuit of an FM modulator along with an oscillation circuit using an emitter-coupled multivibrator, as disclosed in Japanese Patent Publication No. Sho 59-30337. In this oscillation circuit, an oscillating frequency F.sub.o is expressed as follows. ##EQU1## wherein I.sub.o represents the collector current of transistor 1, C represents the capacitance of the capacitor connected between two points A and B, and .DELTA.V represents the crest value of the waveform obtained at the point A or B. Now, assuming that electric potentials at connection points p and q are Ea and Eb, respectively, the crest value .DELTA.V of the voltage obtained at the points A and B is expressed as follows. EQU .DELTA.V=2(Ea-Eb) (2)
Hence, the capacitance C and the crest value .DELTA.V in formula 1 become constants and thus the oscillating frequency F.sub.o is proportional to the collector current I.sub.o.
In FIG. 1, video signals (luminance signals) are applied to an input terminal 2. The value of the electric power source 3 is determined so that the DC potential at points C and D may be equal. A first variable resistor 4 determines the oscillating frequency at no input signal time, that is, the current value I.sub.o in FIG. 1. The current I.sub.o flows to transistor 6 via transistor 5. Therefore, the current I.sub.o also flows to transistor 1 connected to transistor. 6 with a current mirror relationship. As a result, the oscillation circuit oscillates at a constant frequency F.sub.o. When the video signal is applied from the no input signal state (e.g., zero volts), a signal current flows in a second variable resistor 7 and the emitter current of the transistor varies. Thus, a variable current part is added or subtracted to dr from the current I.sub.o and the oscillating frequency is varied. At this time, the voltage at the point D is not changed and hence the oscillating frequency can be precisely changed depending on the video signal.
However, in the frequency control method in FIG. 1, since the variable resistors are required, adjusting work is required in a fabricating process and external parts of an IC and pins are required.